There are known sensors which detect the capacitance, or capacitance change, of a detecting device that is a measuring object, thereby measuring a physical quantity corresponding to the capacitance or the capacitance change. A conventional technique related to such a sensor is disclosed in, for example, Patent Document 1 (Japanese Patent Kokai No. H11-14482 or Japanese Patent No. 3386336).
In the sensor disclosed in Reference 1, an operational amplifier (op-amp) is used to detect capacitance change of a measuring object. The non-inverting input terminal of the operational amplifier is grounded, and the inverting input terminal thereof is connected to the detecting device via a switch, and a feedback capacitor is connected between the output terminal and the inverting input terminal of the operational amplifier. As the capacitance of the feedback capacitor becomes smaller, the output amplitude of the operational amplifier increases, thus increasing the detection sensitivity, but if the capacitance of the feedback capacitor is too small, the output of the operational amplifier cannot exceed the output dynamic range of the operational amplifier, thus becoming saturated. Thus, there is a limit to the increase in the detection sensitivity. In particular, there is the problem that if a feedback capacitor of small capacitance is used to detect changes in the minute capacitance, the output of the operational amplifier will likely become saturated.
Meanwhile, ferroelectric memory that enables high density recording is being studied as one of next generation large capacity storage media. Ferroelectric material such as LiTaO3 can have spontaneous polarization in each micro domain thereof, and the orientation of the spontaneous polarization can be changed by applying an external electric field. Hence, bit information corresponding to the orientation of the spontaneous polarization can be recorded in the ferroelectric memory. Furthermore, by detecting capacitance change of the ferroelectric material, the orientation of the spontaneous polarization can be detected.
For example, in Patent document 2 (Japanese Patent Kokai No. 2004-127489) and Patent document 3 (European Patent Application Publication No. 1398779), there is disclosed a reproducing apparatus capable of reproducing bit information recorded in the ferroelectric memory. This reproducing apparatus utilizes an LC resonant circuit consisting of a polarized portion having minute capacitance (C) of a ferroelectric memory and an inductor having inductance (L). The reproducing apparatus comprises a probe used to apply an alternate electric field to the ferroelectric memory, an oscillator to oscillate at a frequency determined by the capacitance (C) of the polarized portion and the inductance (L), and an FM demodulator to demodulate the output of the oscillator. The resonant frequency of the LC resonant circuit is at about 1 GHz.
Although the reproducing apparatus disclosed in Reference 2 requires an oscillator and an FM demodulator to realize high resolution, the oscillator and the FM demodulator cannot be said to be suitable in configuration for circuit integration. Moreover, there is the problem that the oscillator and the FM demodulator need to operate at a high frequency band of the order of GHz, hence being easily affected by external noise or static electricity.
Further, in order to detect polarization states recorded in ferroelectric memory as disclosed in Reference 2, changes in the minute capacitance of ferroelectric material need to be detected. However, sensors as described in Reference 1 do not have the capability of detecting changes in such minute capacitance.
Patent Document 1: Japanese Patent Kokai No. H11-14482 (Japanese Patent No. 3386336)
Patent document 2: Japanese Patent Kokai No. 2004-127489
Patent document 3: European Patent Application Publication No. 1398779 (Publication of a counterpart European Patent Application of the Japanese Patent Application as provisionally published as patent document 2)